Present sensors for monitoring respiratory effort and other bodily functions generally consist of a flat strap or band that is placed around the torso of a patient and have only one sensing element. These body sensors have two common arrangements for sensing. A first arrangement involves the placement of a single sensing element at one location along a strap, normally at the attachment end. Typically, the sensing element is either a crystal or a ceramic, such as quartz or barium titanate respectively. The strap used is either an elastic-like waistband or girdle-like cloth belt. This arrangement relies upon the strap to transfer all respiratory activities to the point source sensor. The mechanical strains exerted on the strap by the respiratory expansion of the patient's thorax or abdomen are propagated through the strap to the point source sensor which then generates an electrical signal. One problem with this first arrangement is the inflexibility and cost of the sensing element. Crystals and ceramics are typically dense, brittle and stiff. These attributes impede the manufacture of sensors and render the fabrication of complex shapes impractical. The materials are also relatively expensive, resulting in high production costs. Another problem with this first arrangement arises from the reliance upon the strap to transfer torso movements to the point sensor and to provide adjustment means in attaching the sensor to patient. The material which comprises the strap generally dampens the mechanical force signal, thereby resulting in an electrical signal which may not fully reflect the actual respiratory and diaphragmatic efforts of the patient. This dampening effect also increases the likelihood of signal noise and artifacts.
The second common arrangement of body sensors employs a continuous flat strip of sensing material, typically a wire or film, running the length of the strap. The strap used in this arrangement is generally a stiff, belt-like device which, in conjunction with the flat strip of sensing material, maintains a continuous, singular surface contact with the patient. One problem with this arrangement is the prospect that patient position may occlude a large portion of the mechanical force signal, resulting in a weak electrical signal. Another problem is the relatively low signal-to-noise ratio which is the consequence of the inflexible belt-like device, i.e., stiffness in the belt increases the chance that extraneous forces will be exerted upon the sensing element, thereby resulting in relatively large signal artifacts. In applications where flat surface sensors are used, the piezo-film is laminated between two protective surfaces, protecting the thin metallization coating from being damaged or overstressed. However, when the piezo-film is wound on a curved surface, as is done in the structure of this invention, the outer metallization surface is placed under tension. When the thin outer metallization is stressed, the extremely thin film starts to develop invisible hairline fractures or cracks across the width of the piezo-film. Such hairline fractures initially cause an increase in conductive resistance, and eventually can lead to an electrical open or break. It is thus necessary to recognize this potential problem, and to provide an effective solution for it.
Another problem with a belt or strap-like sensor is the method used to attach the sensor and belt around the patient. If the sensor does not contain any elastic means, it is very difficult to provide good adjustment and compliance with the wide variation of body sizes and normal changes around the waist of the patient due to respiratory function. The Spiral-Cord of this invention is a unique structure where an elastic adjustment means is built into each spiral loop, not requiring a separate elastic strap or a separate attachment and adjustment structure.